(1) Field of the Invention
The present invention relates to novel p-amino substituted tetraphenylthiophenes and to electrophotographic photoreceptors containing them. More specifically, it relates to an electrophotographic photoreceptor in which a novel p-amino substituted tetraphenylthiophene is contained as a charge-transporting material in a photosensitive layer on an electrically conductive base.
(2) Description of the Prior Art
Heretofore, inorganic photosensitive materials such as selenium, cadmium sulfide and zinc oxide have been widely used as photosensitive materials for electrophotographic photoreceptors. However, the photoreceptors using such photosensitive materials do not sufficiently satisfy the requirements for electrophotographic photoreceptors such as sensitivity, light stability, moisture-proofness and durability. For example, the photoreceptors using selenium materials have excellent sensitivity but are susceptible to crystallization due to heat or contaminants, so that the characteristics of the photoreceptors deteriorate with use. In addition, this type of photoreceptors has many drawbacks, viz., manufacturing cost is high because of the utilization of vacuum evaporation and it is difficult to mold them into the form of a belt because of poor flexibility. The photoreceptors using the cadmium sulfide material have poor moisture-proofness and durability. Furthermore, the photoreceptors in which zinc oxide is used have unsatisfactory durability.
In order to eliminate these disadvantages of the photoreceptors using the inorganic photosensitive materials, various photoreceptors based on organic photosensitive materials have been investigated.
The photoreceptors developed to remove the above-mentioned drawbacks include function-separated photoreceptors in which a charge-generating function and a charge-transporting function are separately allocated to different materials. In the function-separated photoreceptors, materials having various desirable functions can be selected from a wide range of materials and combined with one another, which permits preparing photoreceptors having high sensitivity and sufficient durability.
Requirements for the charge-transporting material contained in the electrophotographic photoreceptor are as follows:
(1) Possess sufficiently high ability to receive electric charges generated from the charge-generating material.
(2) Capable of promptly transporting the received charges.
(3) Capable of successfully transporting the charges even in low electric field so that no charges remain.
In addition, the photoreceptor must be stable to light and heat during the repeating operation of electrification, exposure, development and transfer, and must have enough durability to obtain faithfully reproduced copy images.
As the charge-transporting materials, various compounds have been reported. For example, poly(N-vinylcarbazole) has been known for some time as a light-conductive material and photoreceptors in which this compound is used as the charge-transporting material have been put into practice. However, this type of photoreceptors has poor flexibility and is brittle, with the result that they readily crack. This fact means that their durability is so poor as to not withstand repeated use. Furthermore, when their flexibility is improved by the use of a binder, electrophotographic properties deteriorate.
Because low-molecular weight compounds have no coating properties, when a photosensitive layer is formed therewith, such a compound is mixed generally with a binder in an arbitrary ratio. Many charge-transporting materials have been suggested which comprise low-molecular weight compounds. For example, hydrazone compounds have high sensitivity as the charge-transporting material. (See Japanese Patent Publication Laid-open Nos. 55-46761, 55-52064, 57-58156 and 57-58157). However, these compounds tend to decompose by ozone generated during corona discharge and they are unstable to light and heat. In addition, when this kind of charge-transporting material is used, the thus-obtained images are poor in contrast or are fogged, owing to the degradation of charge retention ability by repeated use or owing to the accumulation of residual potential, even when initial performance is good.
Many other charge-transforming materials have been suggested but none of them sufficiently satisfy the practical requirements for use in an electrophotographic photoreceptor. Therefore, the development of better photoreceptors is desired.